This invention relates to a method and apparatus for sealing the edges of mailers or other business forms, and more particularly to a cellulosic or polymeric web or sheet product which is coated on its peripheral edges with a bond-enabling material. When toner particles are printed onto the peripheral edges of the sheet over the bond-enabling material, the sheet may be folded and sealed at its edges using the toner as the adhesive by the application of heat and pressure.
In recent years, mailers and other business forms have been developed for printing in high speed impact printers which use ribbons for printing. The forms are typically single-ply continuous forms having variable information printed on one surface of the form. During the manufacture of such forms, hot-melt adhesive is applied at selected marginal edges of the forms for later use in sealing the forms. After printing, the forms are folded and then passed through a device which applies heat and pressure, activating the hot melt adhesive and causing the folded plies to be sealed together along the their common marginal edges.
However, with the advance of microcomputer technology, business forms and mailers are now printed on a wide variety of commercial printing devices which are faster, quieter, and more reliable than traditional mechanical impact printers. Laser printers are an example of a nonimpact printing device, which operate by turning on and off a computer-controlled laser beam of light in a specific pattern to form a latent image of static charges on a paper web. The portion of the web containing the static charges attracts toner particles and forms a toner image, which is then fused on the paper by passing the paper through a pair of rolls which apply both heat and pressure to the paper to bond the toner particles permanently to the paper.
However, mailers and business forms which utilize hot melt adhesives for sealing are not suitable for imaging in laser printers because the high temperatures required to fuse the toner particles to the paper also melt the adhesives on the paper, which then become tacky and jam the hot printer rollers.
An alternative method of sealing in the art involves the use of a self-adhesive or pressure seal made from rubber latex dispersed in water. When applied to a surface and dried, a film is formed which will not bond on contact with paper and other surfaces, but will bond on contact with another film of the same material. These adhesive films are heat resistant and, if properly formulated, do not become tacky to other surfaces when heated. However, formulation of commercially trouble-free self-adhesives has not been completely successful. In addition, self-adhesive seals have a short shelf life of about three months or less.
Recently, a number of attempts have been made to utilize the adhesive properties of toner particles used in nonimpact printers to seal mailers and other business forms. For example, European patent No. 245424 describes a machine for sealing a sheet which has been coated on its peripheral edges with toner particles by a laser printer. The sheet is folded in the form of an envelope and then passed through first and second sealing zones comprising heated, driven pairs of sealing discs which heat the toner particles, causing them to become tacky and adhere to each other, thus sealing the sheet.
Swedish Patent Application No. 8505243 also teaches a sheet material coated with strips of heat-activatable electronically printed toner strips on its edges. The sheet is folded so that the strips come into contact with each other, then the sheet is advanced through a machine having a heating zone which applies heat and pressure to seal the sheet in the form of an envelope.
However, a disadvantage of these methods is that when the toner material is fused to the paper during the laser printing process, it tends to impregnate, or sink into the paper so that there is little toner remaining on the surface of the paper to form a strong bond when the paper is folded and sealed. As a result, the bond formed by the sealing operation is very weak, and the mailer may inadvertently open before it reaches the intended recipient.
Accordingly, the need still exists in the art for adhesively bonding both porous and non-porous surfaces utilizing toner material as an adhesive to provide a strong bond.